1. Field of the Invention
The present invention relates to a thin-type dental magnetic attachment which is embedded in a pit of a denture to hold the denture on teeth roots in the oral cavity using magnetic attractive force.
2. Description of the Related Art
Presently, a dental magnetic attachment which strongly holds and stabilizes the denture on teeth roots by magnetic attractive force is well known. As shown in FIG. 8, its force acts between a dental magnetic attachment 9 in a denture 80 and a keeper 10 made from a soft magnetic material embedded in a root cap 85 inserted into a tooth root 86. The denture 80 has artificial teeth 84, a resin base 83 and a dental magnetic attachment 9. Japanese Patent application Laid open (Kokai) No. 4-227253 discloses a round shaped dental magnetic attachment shown in FIG. 9.
This dental magnetic attachment 9 is composed from a cylindrical shaped magnet 91, a cylindrical shaped yoke 92 having a pit 99 for holding the magnet, and a sealing plate 98 for covering the bottom of the magnet. Here the sealing plate consists of a sealing disk 96 and a sealing ring 97. The yoke, the sealing disk and the keeper are made from soft magnetic materials. The magnet is magnetized to the normal direction to the magnetic attached plane between the dental magnetic attachment and the keeper. When the dental magnetic attachment is set on the keeper, a closed magnetic circuit composed from the magnet, the yoke and keeper is formed. The magnetic flux flows from the top of the magnet, through the yoke, the keeper and the sealing disk back to the bottom of the magnet. The sealing ring made from nonmagnetic material is placed between the yoke and the sealing disk to prevent occurrence of a magnetic short circuit. All of the parts except the magnet are made from corrosion resistant materials.
The sealing disk 96 and the sealing ring 97 are joined to the yoke by laser welding. The laser welding lines are two boundaries between the yoke and the outer side of the sealing ring and between the sealing disk and the inner side of the sealing ring. That means that laser welding is done two times.
However the above invention has some serious disadvantages concerning the sealing method. In the case that the sealing ring has a comparatively large width, the laser welding can be done easily, but the area of the disk becomes so small that the magnetic attractive force decreases. On the other hand, in the case that the sealing ring is narrow, it is very difficult to laser-weld the sealing ring to both the yoke and the sealing disk. Large strain or stress produced by the heat in welding is apt to make some gap or step among the sealing ring, the yoke and the sealing disk. Sometimes cracks occur in the welded parts.
To solve the above disadvantages, another sealing method as shown in FIG. 10 is proposed in the proceedings of the annual meeting of the Japanese Metal Society, at p408 (1996). This dental magnetic attachment is produced by a laser-welding technique of a single application instead of the above laser-welding technique of two times. Its small welded part works as a nonmagnetic sealing ring to contribute an increase in the magnetic attractive force.
In this method, a sealing disk 106 is plated or coated by Ni. It means that Ni coat 105 which is a soft magnetic material is substituted for the nonmagnetic sealing ring. The Ni coated disk is placed on the magnet 101 inserted into the pit 108 of the yoke 102 and then it is laser-welded to the yoke. At this time, Ni coat is melt together with the yoke 102 and the sealing disk 106 made from soft magnetic stainless steel and the nonmagnetic welded part is formed. However this method has some inevitable defects.
It is required that the depth of the welded part is controlled to be just equal to the thickness of the sealing disk. If the depth is not enough as shown by a dotted line (F) in FIG. 10(B) and Ni coat remains, a magnetic short circuit is formed because Ni metal is of soft magnetic materials. Consequently, the magnetic attractive force decreases remarkably. If the depth is too large as shown by a dotted line (E) shown in FIG. 10(B), the heat of laser-welding process damages the magnet so as to cause a decrease of the magnetic attractive force. When a magnet with a small diameter is applied, the magnet is free from this damage, but it is natural that the magnetic attractive force becomes small.
Moreover, the welded part is apt to become an imperfect austenite phase to contain a small volume of ferrite phase. This means that it is difficult to make the welded part nonmagnetic. The reason of the occurrence of the ferrite phase is considered with the Schaeffler diagram which gives the phase constitution after welding stainless steels. A soft magnetic stainless steel of SUS447J1 type with 30% Cr, 2% Mo and residual of iron is used as the yoke material in the above paper. The welded part shows a chemical composition having more than 15% Ni but it has a large part of ferrite phase. This results in a weak magnetic short circuit between the yoke 102 and the sealing disk 106 to make an undesirable decrease in the magnetic attractive force.
If another soft magnetic stainless steel of SUS430 type having 17% Cr and residual of iron is used instead of SUS447J1 type, nonmagnetic welded part would be obtained to give high magnetic attractive force. But the welded part has less than 17% Cr to become very corrosive in the oral cavity.
It is noted that the thickness of the sealing disk is so thin around 0.2 mm, that a laser-welding process with the yoke and the disk is very difficult normally, and yet in the case of the Ni coated sealing disk the laser-welding process is more difficult. Consequently, this dental magnetic attachment has been not yet produced.